Influence of a tension stress field introduced in the elastohydrodynamic contact zone on rolling contact fatigue

The rolling contact fatigue life of modified NU 209 roller bearing inner races subjected to tensile stress mechanically induced in the contact zone, has been investigated using the four-contact fatigue machine.A high tensile stress of 80 MN/m² by a clamp pressure, acting on the inner surface of the specimen, has a catastrophic effect on fatigue life; all specimens burst earlier than pitting failure would have occurred at a near zero tensile stress level.A theoretical study of the influence of the axi-symmetrical stress field, induced by the steady pressure applied inside the hollow rotating cylinder on the stress distribution in the e.h.d. contact zone of cylinders showed a significant increase of the resultant shear stresses caused by tension. The pure shear stress state may exist in the contact zone when the tensile stress level is high.     

Temperature rise due to sliding in rolling/sliding elastohydrodynamic lubrication line contacts: an efficient numerical analysis for contact zone temperatures

An efficient numerical method based on Lobatto quadrature analysis is adopted for a rigorous analysis of temperature in elastohydrodynamic lubrication (EHL) line contacts. Temperature distributions are calculated for maximum Hertzian pressures and rolling speeds varying between 0.5 to 2.0 GPa and 1 to 30 m/s, respectively. Significant mid-film temperature and surface temperature increases have been observed at higher rolling speeds with an increase in loads and slip ratios. Results have been compared with the results of Manton, S. M., O'Donoghue, J. P. and Cameron, A., Temperatures at lubricated rolling/sliding contacts. Proceedings of the Institution of Mechanical Engineers, 1967–68, 182(417), 813–824. An empirical equation is presented for the prediction of non-dimensional maximum mid-film temperature in the contact zone in terms of the dimensionless thermal loading parameter Q, dimensionless load W and slip S, as:

     

Alternation on rolling contact properties of steel due to magnetic field

This paper describes the effects of magnetic field on rolling contact performance of steel/steel disc couple and presents an investigation into the mechanisms governing these effects by applying contact mechanics and magnetism theory.The tests were carried out in disc-on-disc contact configuration under 1.1 T [Tesla] of horizontal static magnetic fields created by permanent magnets in three different orientations.Wear amounts were decreased in the magnetic fields and there was difference in magnetic field orientation. For the characteristic of the surface, results of scanning electron microscope observations point out that finer wear particles and smoother worn surfaces are produced in the presence of magnetic field. The smoother surfaces are also confirmed by surface roughness measurements. For the generation of the finer wear particles, it is considered that subsurface crack initiation point is moved toward the surface due to magnetic field.The effect of magnetic field is considered by calculating the number of cycles required to generate wear particles and the cycle was reduced due to the presence of magnetic field.It is considered, from views of contact mechanics and magnetism theory, that domain walls near the contact region are caught by dislocations when the specimen is magnetised and part of the energy for magnetisation activates the dislocation movement resulting in crack initiation.     

Relative fatigue life estimation of cylindrical hollow rollers in general pure rolling contact

Solid and hollow cylindrical rollers in pure rolling contact have been modelled. The two rollers are subjected to a combined normal and tangential loading. The tangential loading is one‐third of the normal loading value. The finite element package, ABAQUS, is used to study the stress distribution and the resulting deformations in the bodies of the rollers. Then the Ioannides–Harris fatigue life model for rolling bearings is applied on the ABAQUS numerical results to investigate the fatigue life of the solid and hollow rollers. Using the fatigue life of the solid rollers as the reference fatigue life, the relative fatigue lives of hollow rollers are determined. Four main different hollowness percentages are been studied: 20, 40, 60 and 80%. The hollowness percentage is the ratio of the diameter of the hole to the outer diameter of the cylinder. For each of those hollowness percentages, two cases are studied – when the two rollers in contact are hollow and when one hollow roller is in contact with a solid roller. This study includes two main models: Model 1, where the two cylindrical rollers in contact are of the same size, and Model 2, where the two rollers in contact are not of the same size. The estimated relative fatigue lives of hollow rollers showed a great improvement of the fatigue life compared with solid rollers under the same loading conditions. This was a result of the redistribution of stresses in the contact zone in the case of hollow rollers. Redistribution of stresses over a larger volume of the roller body decreased the peak stress and reduced the volume under risk. Increasing the hollowness percentage from 20 to 60% increased the flexibility of the roller, and better stress distribution was achieved, which resulted in improving the fatigue life. Although 80% of hollowness rollers have more flexibility than 60% of hollowness rollers, the bending stresses (σ_b) on the inner surface of the rollers tend to decrease the fatigue life. Copyright © 2007 John Wiley & Sons, Ltd.     

Residual stress in some elasto-plastic problems of rolling contact with friction

The problem or rolling contact is of a great interest from the practical and cognitive points of view. It concerns for example a wheel passage on a rail, as well as many other processes where rolling or rolling with sliding is involved. Particularly, residual stresses distributions in the subsurface area due to plastic deformations are of significant importance: these stresses strongly influence fatigue limit and cracking tendencies of the material. A motion of the wheel on the rail (or of a roller on a strip) often has a very complex character: it consists of rolling, sliding in the longitudinal direction due to brakings and accelerations, sliding in the transversal direction, etc. Investigations conducted on this field by many authors have not yet delivered a unique solution. The aim of this paper is to determine the influence of some of these components of the wheel motion (separately and in common action) on residual stresses distributions. The problem has been investigated numerically by the FEM method, using the program SEGLA developed by the authors. The program can be applied to solving linear and nonlinear 2D, axisymmetric and 3D problems with elasto-plastic effects in the nonlinear contact and fracture mechanics. It uses three, four, six, eight and nine-node elements and also the special “spring” elements to model contact. The applied algorithm allows automatic analysis of these fragments of boundary, which enter and leave the contact; in the area where the “new” contact is created the program introduces additional elements.     

The effect of tufftriding on the rolling contact fatigue behaviour of low alloy steel cylindrical specimens

Investigations of the rolling contact fatigue behaviour of a low alloy steel are reported. Experiments were conducted on untreated and Tufftrided cylindrical specimens using Tellus 15 oil and contact stress-life curves were obtained. Both curves exhibit knee points with well-defined pitting limits. Tufftriding increases the pitting limit substantially. Wear loss studies indicated that Tufftriding leads to negligible wear loss under high contact stresses. Microhardness explorations established that strain hardening is exhibited by worked untreated specimens but no strain hardening was found with worked Tufftrided specimens. Tufftrided surfaces thus retain their original ductility and this contributes to their better performance under rolling contact fatigue conditions.     

On the application of Dang Van criterion to rolling contact fatigue

In this note, the problem of the calibration of the Dang Van multiaxial fatigue criterion is addressed. The discussion is based on uniaxial fatigue tests performed with different stress ratios. Results show that the usual technique for calibrating the constants of the Dang Van criterion does not agree with experimental evidence, especially for negative stress ratios. For this reason, a different fatigue failure locus made of two straight line segments is proposed and typical three-dimensional rolling contact stress histories are analyzed using the traditional and proposed methods. Results show that the conventional technique does not agree with knowledge coming from shakedown approaches of rolling contact while the proposed method seems to constitute a more appropriate limit.     

Mechanics and behaviour of hollow cylindrical members in rolling contact

The mechanics of contact of hollow cylindrical elements were studied. The maximum contact stress is less for a hollow cylinder than for a solid cylinder. A simple numerical method is proposed to estimate the reduced contact stress by finding an equivalent modulus of elasticity for the hollow cylinder for use in the contact stress equation.Rolling contact fatigue experiments conducted on hollow and solid cylindrical specimens of the same material and same external dimensions revealed that the hollow specimens performed much better than the solid specimens, a result which can be attributed to the lower contact stress in hollow specimens. Pitting was the mode of failure for all the tested and failed specimens. Microscopic studies revealed that in many of the pitted hollow specimens flexural fatigue cracks had initiated at the inner surface.     

Changes in the micro-geometry of a rolling contact

The effect of the changes in the micro-geometry in a rolling contact on wear processes like running-in is of great interest. Recently, a wear measurement system-based on the comparison of local changes on the surface topography in a rolling contact has been developed. Some results of the rolling experiments are presented. In particular, the influence of surface roughness, asperity layout and hardness on deformation mechanisms is studied. The shakedown and deformation mechanisms in rolling contacts are discussed.     

The effect of variable amplitude loading on stress distribution within a cylindrical contact subjected to fretting fatigue

A finite element model of a cylindrical Hertzian contact on a test sample subjected to alternating shear loading has been developed. The model has been used to investigate shear stress distributions at the contact during variable amplitude fretting fatigue for a load configuration in which the sample cyclic stress is applied in phase with shear force on the cylindrical contact. It has been found that during constant amplitude cyclic loading, shear stress distributions and positions of the stick-slip boundary at load maxima and minima remain fixed. Application of overloads changes the stress distribution and the position of the stick-slip boundary attained by loading of subsequent cycles. The largest cycle maximum stress determines the position of the stick-slip boundary adopted by subsequent smaller amplitude cycles. In general variable amplitude fretting fatigue the position of the stick-slip boundary will be changing with each load cycle. Hence fatigue initiation processes will occur at locations dispersed over an extended region over the contact. The implications of this behaviour for models for fretting fatigue life calculation are explored.     

Influence of a ring flat zone in the point contact surface on thermal elastohydrodynamic lubrication

This paper describes a geometrical profile, an elastohydrodynamically lubricated point contact surface with a ring flat zone, aimed at building up local line contact elastohydrodynamic lubrication (EHL) in point contact conjunctions to reduce the influence of side-leakage on the central film thickness. Effects of the ring flat zone on the thermal EHL characteristics are studied. A dimensionless coefficient, r_W, is defined to represent the relative half width of the ring flat zone in a point contact EHL surface. Thermal EHL numerical simulations have been performed to investigate the influence of r_W on the film thickness as well as pressure, temperature and friction coefficients under different operating conditions. In the range of 0≤r_W≤1.0 results show that the minimum film thickness decreases with increasing r_W and the central film thickness increases with increasing r_W, and the influence of r_W on the film thickness is more pronounced than those on the maximum pressure, the maximum temperature and the friction coefficients. It is revealed that the proposed ring flat zone with appropriate width is beneficial to the thermal lubrication.     

The role of nitrogen content on the rolling contact fatigue performance of En31 ball bearing steels

Using an accelerated service-simulation test, the role of nitrogen content on the rolling contact fatigue resistance of a range of En31 ball bearing steels was assessed. Nitrogen content appears to be a significant factor in steel performance. Air-melted neutral open hearth and acid open hearth materials, which were superior to air-melted basic electric arc steels, were comparatively low in nitrogen. With the basic electric arc materials there was a trend towards reduced rolling contact fatigue life with increased nitrogen content.     

The use of artificial defects in the 5-ball-rod rolling contact fatigue experiments

The fatigue performance of steels under rolling contact loading is normally evaluated using time consuming rigs, though accelerated tests are also used. The present work reports the analysis carried out on tests accelerated by using artificial defects (ADs) over the wear track. Two modifications were introduced to the 3-ball-rod testing protocol. The first modification reduces the time required to produce a spall by substituting 5-ball for the traditional 3-ball mounted on a retainer. The second modification is intended to study surface resistance to rolling contact fatigue (RCF) by furnishing the wear track with artificial defects which promote surface nucleated failures. Three different types of pedigreed artificial defects, i.e. Knoop, Vickers and Rockwell-C indentations were used. The experiments were conducted on specimens of AISI 440C. The failures obtained were optically analyzed and the exact location for spall nucleation was determined to be inside the AD rather than outside. An attempt was then made to correlate the results with the specific oil film parameter (lambda) but life changes could not be predicted by this criteria. The results indicate that the combination of ADs in conjunction with the five balls methodology does not bias the results, but it considerably reduces the time required to test materials.     

弹流润滑高速角接触球轴承旋滚比分析

为了提高主轴轴承的高速性能,提出弹流润滑高速角接触球轴承的数学计算与分析模型.针对实际生产工况建立弹流润滑高速角接触球轴承分析计算模型.并将弹流润滑高速角接触球轴承的旋转打滑分析转化为旋滚比的分析.通过计算与分析,得出弹流润滑高速角接触球轴承在不同工作条件下或选用不同轴承参数时,轴承旋滚比的变化规律及旋滚比与各参数之间的关系.为实际生产中高速角接触轴承的选择使用和轴承的参数优化设计提供了理论依据,并为机床高速主轴用支撑轴承的设计与优化提供分析计算模型.     

The effect of changing the rolling direction on the rolling contact fatigue lives of annealed and case-hardened steel rollers

The effects of changing the rolling direction and of repeated loading on the rolling contact fatigue lives of annealed 0.45% carbon steel rollers and case-hardened nickel-chromium steel rollers under conditions of sliding rolling contact were studied. The influence of plastic flow in the subsurface layer on the rolling fatigue life was examined. The increase in the rolling fatigue life of an annealed steel roller due to a change in the rolling direction was significant, especially when the rolling direction was changed just before the formation of macroscopic surface cracks and pits. The effect with case-hardened steel rollers was negligible. The varying effects of changing the rolling direction on the rolling fatigue life were due to differences in work-hardening and the extent of plastic flow in the rollers.     

Review of the damage mechanism in wind turbine gearbox bearings under rolling contact fatigue

Wind turbine gearbox bearings fail with the service life is much shorter than the designed life. Gearbox bearings are subjected to rolling contact fatigue (RCF) and they are observed to fail due to axial cracking, surface flaking, and the formation of white etching areas (WEAs). The current study reviewed these three typical failure modes. The underlying dominant mechanisms were discussed with emphasis on the formation mechanism of WEAs. Although numerous studies have been carried out, the formation of WEAs remains unclear. The prevailing mechanism of the rubbing of crack faces that generates WEAs was questioned by the authors. WEAs were compared with adiabatic shear bands (ASBs) generated in the high strain rate deformation in terms of microstructural compositions, grain refinement, and formation mechanism. Results indicate that a number of similarities exist between them. However, substantial evidence is required to verify whether or not WEAs and ASBs are the same matters.

     

Macroscopic and microscopic changes in the surface layers of annealed and case-hardened steel rollers due to rolling contact

Measurements of the residual stress and observations of the microstructure at the surface and in the subsurface of rollers were performed during rolling contact fatigue tests of annealed 0.45% carbon steel and case-hardened nickel-chromium steel rollers. Compressive residual stresses in annealed rollers were induced by the rolling contact. With case-hardened rollers they were induced by heat treatment prior to the rolling contact fatigue tests. After the rolling contact fatigue tests the compressive residual stresses on the surface of the annealed rollers and in the subsurface of the case-hardened rollers relaxed; a characteristic substructure was formed by the stress cycles, which caused surface failure. It was confirmed that the microcracks leading to surface failure initiate on the surfaces of annealed rollers and in the subsurface of case-hardened rollers.     

A test method to study the change in surface topography due to running‐in of a rolling contact

A method is proposed to determine the change in surface topography during running‐in of rolling contacts. Two types of experiments have been conducted in the present work to study the running‐in of the pure rolling contact situation: repeated moving and general free rolling using a high accuracy measurement setup. The results show that the surfaces run‐in with the proposed repeated moving contact method gives a fast running‐in when compared with the general free rolling contact method. The proposed repeated moving contact method appears to be a good method to study the ideal or pure rolling contact situation because wear, as present in the general free rolling contact method, is avoided. Copyright © 2013 John Wiley & Sons, Ltd.     

The topography of the field and flux inside and above the surfaces of ferromagnetic plates during their contact and contactless magnetization

Modeling and experimental studies of the spatial distributions of the field and flux inside and above the surface of ferromagnetic plates of different dimension types, which were locally magnetized by U-shaped electromagnets, were performed. It was established that the location of a magnetic inhomogeneity in the interpole zone of an electromagnet substantially affects the results of a local measurement of the coercive force using a demagnetization current. It is shown that the presence of a gap in the magnetic circuit impairs the magnetization of the interpole zone of an object to a higher degree than the magnetization of the near-pole zone. Recommendations on the concentration of the magnetic flux in the interpole zone via a decrease in the interpole distance of the electromagnet are given. Possible locations of internal-field probes that provide local measurements of the magnetic properties of a substance are determined.     

Effects of molybdenum disulphide and an organic molybdenum compound in gear oil on rolling contact fatigue

Sliding-rolling contact fatigue tests with roller pairs were performed using three kinds of oils as lubricants in order to elucidate the effects of molybdenum disulphide and an organic molybdenum compound in oil on fatigue life and surface failure. A Weibull analysis was applied to pitting failure to analyse the depths and sizes of pits. These experimental and analytical results for each oil were discussed with respect to the amplitudes of the ratios of shear stresses to Vickers' hardnesses beneath the roller surfaces calculated considering friction forces and hardness distributions at elevated temperatures in service.